The invention relates to a guide shoe for a guiding device of a clamping unit of a plastic processing machine, in particular for a clamping unit of a two-platen injection molding machine with tension columns. The invention is described below by way of example by means of an injection molding machine. However, it is not restricted to this, and can be transferred to other plastic processing machines which have a linearly guided clamping unit.
In injection molding machines, the two mold halves of an injection mold are received respectively via a fixed mold platen and via a moving mold platen, which during the injection molding cycle are moved towards one another or away from one another. The fixed mold platen is fastened here in a stationary manner on the injection molding machine. The opening and closing of the mold is brought about via the moving mold platen. The direction of movement of the moving mold platen is therefore designated as the clamping direction of the injection molding machine and runs in longitudinal direction thereof. If is often also defined as X-direction or as X-axis of the injection molding machine and of the injection molding components, as it also represents the (main) demolding direction for the injection molding process. The Y-axis runs accordingly horizontally transversely to the clamping direction, so that the vertical axis of the injection molding machine represents the Z-axis.
The closing of the mold and the application of the closing forces for the mold takes place in modern injection molding machines via tension columns which have the task of transferring the very high forces for closing the tool onto the mold.
In modern injection molding machines, the tension columns have no guiding function for the mold platens and they are “merely” responsible for applying the closing forces. For the guidance of the moving mold platen, guiding devices are usually provided, on which great requirements are placed with regard to accuracy of guidance and loading capacity.
For a short cycle time of the injection molding process, it is important to be able to move the moving mold platen to and fro in X-direction at as high a speed as possible between the opened and the closed positions of the injection mold. For this, the moving mold platen is usually guided in clamping direction via guide rails which are uncoupled from the linear drive. In this respect, great requirements with regard to ease of mobility and rigidity are placed on the guide rails or respectively on the guide shoes which slide on the guide rails, so that the moving mold half can be guided in a parallel manner on the moving mold platen or respectively on the mold platen with the maximum precision and at high speeds and at high accelerations. For this, in addition to a low friction of the sliding- or rolling elements of the guide shoes on the guide rail, also a high degree of rigidity of the guide shoes is necessary with respect to bending/tilting about the Y-axis and in Z-direction, wherein moving masses are to be minimal.
The high acceleration values are reached especially on the transition from the open position of the tool into the fast motion for the drawing near of the two mold halves and on braking from the fast motion into the actual closing movement and in the corresponding reversed movement sequences.
Shortly before reaching the closed position, the moving mold half is braked, so that a misalignment, which is possibly present between the two mold halves, must be compensated by suitable compensating means in the non-fast motion. This braking in fact causes high tilting moments of the moving mold platen about the Y-axis with respect to the guides, which can be received by rigid guide shoes. For this, these usually have an elongated shape in X-direction.
With a compensating of any minimal misalignments which may be present between the two mold halves in Y- and/or in Z-direction, tensions occur in the mold platens, which in the case of the moving mold platen are transferred to the guiding devices if no possibility for compensation is provided for this.
After reaching the closed position, i.e. still before the mold is filled, the closing force is applied onto the two mold platens, so that the two mold halves are not pressed apart by the filling of the mold. Here, the mold platens are deformed minimally under the effect of the closing force. The deformation has its greatest effect on the marginal regions of the platen.
The above-mentioned deformation of the mold platens results on the one hand from the applied closing force and proportionately also from the applied injection pressure on injecting of the plasticized plastic material.
Therefore, a deformation of the mold platens is understood below to mean a minimal elastic deformation (lying in the hundredths and tenths of a millimeter range) by application of the closing force and/or of the injection pressure.
By introduction of the plastic material with injection pressure for filling the mold, the two mold platens are deformed minimally by the high injection pressure necessary for this. Injection pressures of 1000 bar and above are entirely usual here. The convex deformation of the mold which occurs here is passed on to the mold platens, wherein these deformations, in particular on the moving platen, are usually transferred, i.e. in the prior art, to the guiding elements, i.e. to the guide shoes and for the most part also to the guide rails. However, according to the conventional prior art deformations on the guiding elements are to be avoided, so that a good linear guidance is achieved. This has the result that the guiding elements are accordingly constructed to be rigid and robust and therefore generally very solid, so that the deformations on the moving mold platen and on the guiding elements are kept small, in order to thus avoid damage to the guiding elements, in particular to the guide rails.
Only an exact as possible parallelity of the two mold platens and hence of the two mold halves can ensure an optimum part quality, a high part number with short cycle times. Furthermore, a high service life of the injection mold and also of the injection molding machine can only be achieved when the two mold halves are held precisely parallel to one another. The better the parallelity of the mold platens can be maintained, the lower is the tool wear and the accompanying damage to the tool and to the guiding elements.
A great importance is therefore given to the guiding elements for moving mold platens, which are usually arranged in Z-direction beneath the moving mold platen and are present once in each case per side on the injection molding machine. They must guarantee a high movement speed of the moving mold platen and at the same time be able to receive high tilting moments on accelerating or respectively braking of the mold half. In addition, they must receive tensions which can occur when the two mold halves draw close to one another shortly before and in the closed position owing to small misalignments of the two mold halves. Furthermore, the guiding devices must be able to receive deformations of the moving platen which occur when closing force is built up and the mold is acted upon with injection pressure. All these stresses are to be received by the guiding elements so that damage, in particular to the sliding/rolling elements and/or to the guide rail, is avoided.
A mold clamping device is known from DE 41 41 259 A1, which can compensate small misalignments between the mold halves in closed position and also transversely to the clamping direction and in height direction, i.e. in vertical direction, by a limited displaceability of the moving mold platen.
The moving mold platen is mounted here in a floating manner on a machine slide, such that the moving mold platen has a small freedom of movement in longitudinal direction of the injection molding machine or respectively in vertical direction of the injection molding machine. A compensation in transverse direction takes place by deformation of pressure bolts, which support the moving platen in Z-direction. Through this arrangement, tensions on closing of the two mold halves are avoided, because the two mold halves are moved towards each other in an almost parallel manner by the floating mounting of the moving mold half.
However, an equalization or respectively a compensation of the tensions which occur under closing force and on filling of the mold by the injection pressure, is not possible with the device according to DE 41 41 259 A1.
In DE 44 03 079 C1 a possible inclination about the Y-axis of the moving mold platen is compensated by a compensating mechanism in the machine slide. For this, the machine slide has guide rollers mounted in an oscillating manner, which compensate the inclination.
Also in this possibility for compensation, a compensating of the tensions which occur by closing force and on filling of the mold by the injection pressure, is not possible. In addition, this device does not provide any compensation for misalignments in the Y- and/or Z-direction.
For compensating the deformations of the moving mold platen on introducing the injection pressure onto the closed mold, DE 196 08 135 A1 proposes preferably configuring the moving mold platen from a thin clamping part and a thicker force transmission part, so that deformations which are brought about by the injection pressure, are absorbed by the thinner layer of the mold platen. In so doing, the thinner layer of the mold platen deforms, which is connected by pressure elements in a decoupled manner with the thicker force transmission layer of the moving mold platen. This complex configuration of the moving mold platen results not only in high manufacturing costs for the injection molding machine, but requires furthermore a solid and expensive configuration of the moving mold platen, whereby high acceleration- or respectively deceleration forces are necessary.
This, in turn,—if one does not wish to accept any extension to the cycle time—leads to substantially greater and stronger drive units, which additionally increase the costs of the injection molding machine and impair the energy balance.